One of the advantages of magnetic (MRI) diagnostic imaging is that in addition to imaging data the MRI equipment can be used for acquiring spectroscopic data which may be graphically displayed. The spectroscopic data reveal specific clinical information about biological components. Magnetic resonance was used in spectroscopy even prior to the advant of magnetic resonance diagnostic imaging equipment. In magnetic resonance spectroscopy, the magnetic resonance equipment is used for providing data and/or graphical (spectrum) displays rather than an image of the patient.
In both magnetic resonance spectroscopy (MRS) and in MRI the patient is subjected to a very large static magnetic field. Certain types of nuclei, such as protons i.e. hydrogen nuclei, phosphorus nuclei, carbon nuclei align themselves as "spins" with the static magnetic field on a statistical basis to provide a net magnetic force "M" in the direction of the large static magnetic field. The aligned spins precess about the axis of the static magnetic field at the Larmor frequency. In MRI or in-vivo MRS, gradient fields are applied to localize the effects of a radio frequency Rf pulse. A radio frequency pulse having a frequency equal to the Larmor frequency of a particular element will cause the aligned spins of that element to nutate, i.e. rotate toward a plane transverse to the axis of the static magnetic field.
The Larmor frequency is defined as: EQU f=.gamma.Bo/2.pi.where:
Bo--is equal to the strength of the static magnetic field, and
.gamma.--is the gyromagnetic constant for each element.
When the Rf pulse is removed then the nutated spins rotate toward the original aligned position. The component of the nutated spins in the transverse plane defocuses and while precessing it generates a signal known as the free induction decay (FID) signal. It is this signal which is detected and used for providing the spectroscopic data which may be graphically displayed. In in-vitro MRS where the samples are small the whole sample may be subjected to an Rf pulse. In in-vivo MRS where the samples are larger a magnetic gradient pulse is supplied simultaneously with the Rf pulse to localize the Rf pulse.
If a complete volume of a sample is subjected indiscriminately to the Rf pulse then the spectroscopic data reveals quantitatively spectroscopic information on all of the elements or nutated within the sample. However, for diagnostic purposes it is imperative to know the location of the energized or nutated elements. Thus location information is required in addition to the quantitative spectroscopic data. For example, if a woman's breast is being examined, for diagnostic purposes by phosphorus MRS it is important to know precisely the region in the breast from which the spectroscopic data was obtained. It is also imperative to find areas where there is no phosphorus, since high concentrations of certain phosphorus compounds in a breast may be an indication of malignancy in the breast. A comparison of a section of the breast containing phosphorus compounds with the section that does not indicate phosphorus is important to assure the reliability and integrity of data obtained.
Accordingly, it is important to obtain data on more than one section of the patient being examined. For the purposes of this invention the sections may be planes or volumes.
In the past to obtain spectroscopic data on different sections of the patient, a first scan sequence was performed to obtain the data from a first section and subsequently a second scan sequence was performed to acquire the data from the second section. If data from ten sections were desired then ten different scan sequences were run. In spectroscopy, where the data is obtained from elements, such as phosphorus, sodium, or carbon, which are much less prevelant than hydrogen the signals are extremely weak. Accordingly, to enhance the signal-to-noise ratio, it is customary to run between 100 to 1000 scans of the same section to obtain averages that give a sufficiently improved signal-to-noise-ratio to provide useful data. Thus, where a plurality of sections were scanned the time required for the complete scan in the prior art was the basic scan time multiplied by the number of scans of the same section necessary for averaging purposes, multiplied by the number of sections from which data was required.
It has always been an aim of the scientists involved in developing magnetic resonance equipment for diagnostic purposes whether for use in imaging or spectroscopy to decrease the time required for data acquisition. The reasons for the continuing attempts to decrease the data acquisition time are fairly obvious. From a practical stand-point the less time required for each scan the more patients can be put through the equipment. Obviously, the more patients that can be examined using the equipment the more the cost per patient will be reduced. In addition a patient's comfort is always a major consideration. The longer the patient has to be immoblized for examination the greater the patient's discomfort. Also it should be understood that not only it is comfort of the patient involved, but also the results. The longer the test takes the more chance there is of voluntary or involuntary movement by the patient.
Accordingly, it is a prime objective of scientist in the magnetic resonance diagnostic field to increase throughput and decrease the scan time per patient.
More particularly, it is an object of the present invention to provide a method of acquiring in-vivo spectroscopic data from multiple regions of a patient in substantially the time required to acquire in-vivo spectroscopic data in a single region.